1. Field of the Invention
The present invention relates to a dimensionally stable electrode for treating hard-resoluble waste water.
2. Description of the Related Art
Waste water such as industrial waste water that has many different properties and shapes, high concentration of organic compounds and sometimes strong colors has negative effects to the environment should it not be treated properly. Specifically, the hard-resoluble compounds are extremely slow to decompose because of its complicate molecular structure, but also nearly removable by conventional active slurry process, biochemical, chemical or physical method, or multi-processing thereof because of its poisonous property. Therefore, in the case of treating waste water containing hard-resoluble or harmful organic compounds by slurry processing, a proper pre-treatment technique capable of securing process stability, reducing the load imposed on an aeration container, and increasing bioanalysis yield are essential. So far, Electrochemical or ozone oxiding method has been widely known as an effective method for treating such waste water.
There are many patent applications with regard to the electrochemical method such as Korean Patent application titled Multi-step electrolytic agglutination system (application number: 98-001790), Method for treating dyeing waste water and pigments waste water using electrolysis and heat-energy (application number: 97-020062, publication number: 98-001852), Reagent for treating dyeing waste water using electrolysis (application number: 96-022480, publication number 98-001851), Apparatus for decoloring dyeing waste water (application number: 96-018074, publicaiton number: 97-074671), Method for decoloring dyeing waste water using chroline compound and electrolysis (application number: 96-014619, publication number 97-074670), carbon electrode using a ceramic binder (application number: 97-033560) and so on.
The electrochemical systems disclosed in the above applications comprised of anode(s) on which oxidation occurs and cathode(s) wherein reduction occurs. The anodes used for oxidation reaction can be divided into soluble anode and non-soluble anode.
The soluble anode is inexpensive but it is hard to purify because second contamination occurs thereon in a large quantity.
Most common non-soluble anode is titanium substrate coated with platinum-family oxide(s). Electrodes for treating waste water requires higher oxygen overvoltage than chlorine generating voltage.
Since conventional electrodes have low chlorine overvoltage, Cl--is easily oxidized to be chroline. The chroline easily reacts with the waste water to secondarily contaminate it. Furthermore, a catalyst for high oxygen overvoltage such as Ru, Pt and Ir is expensive. Especially, an electrode composed of platinum-system catalyst is subject to the following reaction (1) and therefore, its activity disappears. EQU RuO.sub.2 (s)+O2RuO.sub.4 (sol) (1)
Wherein, "s" represents solid state and "sol" represents the state of being gushed from the solution.
When the RuO.sub.2 starts to gush, the electrode starts to be damaged, the titanium substrate is subjected to the corrosion and finally, holes occur thereon. Therefore, the life time of the electrode is extremely short.
Incidently, when electrolytic container system stops, PbO.sub.2 electrode, which is the second most commonly used electrode, is reduced to be Pb by an instant reverse reaction. Such gushed Pb gives rise to another contamination.
An electrochemical electrode requires the satisfaction of the following three conditions.
(1) cost PA1 (2) voltage PA1 (3) life time
The present invention provides for an electrode that satisfy the above mentioned three conditions.
The oxides of Ru, Ir, Pt, Ti, Pb, etc. can be used as an electrochemical catalyst. However, because such substances are expensive, a substitute electrode is required.
Sn can be considered as an electrode substance for treating waste water according to the present invention, because it is non-platinum type, relatively cheap and has higher oxygen generating volatage than chlorine generating voltage.
The two factors affecting the durability are (i) oxidation-resist property of a substrate and (ii) compatibility of an electrode. Titanium, a common substrate, is susceptible to be easy oxidation and reduced by oxygen and hydrogen respectively. Therefore, titanium oxide(TiO.sub.x) having strong resistance to oxidation and reduction can be considered as a substitute of Ti. Herein, x is between 1.1 to 2, more preferred, between 1.1 to 1.9, and most preferred, between 1.5 to 1.8.
Considering durability, an electrode catalyst requres higher oxygen overvoltage because platinum-family oxides are oxidized by oxygen to be dissolved. As such catalyst, there are TiO.sub.2, PtO.sub.2, carbon, RuO.sub.2, SnO.sub.2 and so on, each of which has good adhesiveness.
With regard to voltage, the electrode catalyst substance having high oxygen overvoltage should be selected. Among conceivable substances, SnO.sub.2 is most preferable. If the content of SnO.sub.2 is rapidly increased, conductivity also rapidly increases. However, If the content of SnO.sub.2 is rapidly increased, at the same time, SnO.sub.2 is relatively easily oxidized to be gushed into an electrolyte.
Therefore, considering the characteristics of each electrode catalyst, a two-element or three-element catalyst system is desirable. For example, a catalyst can be considerable which is based on SnO.sub.2 having relatively good durability, reasonable price and desirable voltage, and contains more than one additive element capable of intensifying durability.
In case of two-element system of TiO.sub.2 and SnO.sub.2, if content of SnO.sub.2 is over 90 mole %, oxygen overvoltage rapidly decreases and therefore, the durability of the electrode is deteriorated.
With respect to conductivity alone, PtO.sub.2 can be considered but it has a problem of stability. Also, carbon has a problem in compatibility with metal elements.